1. Field of the Invention
The present invention relates to an optical semiconductor device and a method of manufacturing the same and more particularly to an optical semiconductor device having a high output semiconductor laser used for a light source of an information processing unit such as an optical printer and an optical disk unit, a semiconductor laser amplifier used for light amplification, an optical active element such as a semiconductor laser used in a transmitter of an optical communication apparatus or a photodiode used in a receiver thereof and an optical waveguide, and a method of manufacturing the same.
2. Description of the Prior Art
With the advance of semiconductor communication technique, the production technique of a semiconductor laser is also improved, and researches in integration of a semiconductor laser with another optical semiconductor element are being made extensively in recent years. For example, an apparatus in which a DFB (distributed feedback) laser and a light modulator are integrated and an apparatus in which a DBR (distributed Bragg reflector) laser and a mode transducer (a beam size converter) are integrated are available.
A mode transducer is a mechanism for narrowing an output beam of a semiconductor laser originally having an output angle as wide as 30 to 40 degrees, and for facilitating optical coupling in case a semiconductor laser and an optical fiber are formed into a module.
In a semiconductor laser, the more intense the optical confinement of an optical waveguide is, i.e., the smaller a light spot diameter is, the smaller an oscillation threshold becomes, and a luminous efficiency is improved. As the light spot diameter gets smaller, however, coupling with the optical fiber becomes more difficult.
Further, a semiconductor laser requiring a high output such as a laser for exciting a fiber type optical amplifier using an optical fiber doped with erbium or a semiconductor laser for writing information in a optical disk has such a problem that an optical power density rises at a laser end face and damage of the end face is liable to be produced. Furthermore, a semiconductor laser amplifier has such a drawback that the optical output is saturated easily when beam confinement is intense.
Those photodiodes that have a rapid speed of response and a high quantum efficiency are required, and furthermore, those that can be formed into a thin shape and in that electrical wiring is easy are demanded.
An end face incidence waveguide type photodiode is available as a photodiode which meets such a requirement. In this waveguide type photodiode, the more intense the beam confinement is, the shorter the waveguide is made. With this, internal pn junction capacity thereof is reduced, thus making high-speed response possible. Moreover, reactive components of light absorption such as free carrier absorption of a cladding layer and the quantum efficiency is increased, thus improving sensitivity.
Under such circumstances, a semiconductor laser, a semiconductor laser amplifier and a photodiode having an optical waveguide in which beam confinement is intense inside and beam confinement is weak at an end face are demanded in the fields of optical communication and optical information processing.
So, a waveguide for converting an optical beam diameter composed of a semiconductor has been proposed as shown in FIG. 1A to FIG. 1C. The semiconductor waveguide shown in FIG. 1A is disclosed in [1] the Institute of Electronic Information Communications in Japan, National Autumn Meeting 1992, Lecture Number C-201 for example. The semiconductor waveguide shown in FIGS. 1B and 1C has been proposed in [2] the Institute of Electronic Information Communications in Japan, National Autumn Meeting 1992, Lecture Number C-202 for instance. In [3] T. L. KOCH et al., IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 2, NO. 2, 1990, the semiconductor waveguide is proposed in a mode transducer integrated Fabry-Perot semiconductor laser (hereinafter referred to also as an FP-LD) having a waveguide for converting an optical beam system.
An optical beam diameter converting waveguide shown in FIG. 1A has an InGaAsP core layer 3 surrounded by an InP substrate 1 and an InP cladding layer 2 at top and bottom and left and right.
The lateral width of the InGaAsP core layer 3 is wide at one end face and gets narrower as getting near another end face, and the optical beam diameter spreads in a lateral direction on the side where the width is narrowed in accordance with the variation of the width. The thickness of the InGaAsP core layer 3 is uniform, and the plane pattern thereof is formed by lithography technique using an exposure mask.
Thus, the semiconductor waveguide is effective for converting an optical beam diameter in a lateral direction, but the optical beam diameter in a longitudinal direction (a thickness direction of the layer) is not converted.
Now, in an optical waveguide using a semiconductor, since the optical beam diameter in the thickness direction of the core layer is generally smaller than that in the width direction thereof, conversion of the optical beam diameter in the longitudinal direction is important for the improvement of the optical coupling efficiency with an optical fiber, an optical semiconductor element or the like. Since the optical beam diameter in the thickness direction of the layer is not converted in the semiconductor waveguide for converting the optical beam diameter shown in FIG. 1, a significant effect cannot be expected for the improvement of the optical coupling efficiency between the semiconductor optical waveguide and an optical fiber or the like.
In order to increase the coupling efficiency, it is conceivable to form the semiconductor waveguide for converting the optical beam diameter thereof and the waveguide-shaped photodiode into an integral construction. Since the thickness and the composition of these core layers become uniform in the optical axis direction, however, the end face of the semiconductor waveguide also becomes a light absorption region and the optical loss is increased. Furthermore, since a PN junction is exposed at the end face, a dark current is increased. When the dark current is more or less increased, there is a problem that a signal-to-noise ratio is deteriorated in case very high sensitivity is required.
As against the above, the semiconductor waveguide for converting the optical beam diameter shown in FIGS. 1A and 1B has a construction that the optical beam diameter is converted in the thickness direction of the film.
This semiconductor waveguide has a first InP cladding layer 5 laminated on an InP substrate 4, a multi-quantum well (MQW) layer 6 composed of an InP well and an InAlAs barrier formed thereon and a second InP cladding layer 7 formed on the MQW layer 6. Further, an InGaAsP core layer 8 is formed in the MQW layer 6, and the film thickness at one end thereof is thinned in the MQW layer 6. Besides, the MQW layer 6 serves as a cladding layer for the core layer 8 and as a core layer for the first and the second InP cladding layers 5 and 7.
The core layer 8 is formed both in a gain region 9 and a mode conversion region 10. Further, the core layer 8 in the mode conversion region 10 is formed in a tapered shape in the thickness direction, and gets thinner as becoming more distant from the gain region 9.
The light advancing in such a semiconductor waveguide is confined in the MQW layer 6 and further confined more intensely in the cladding layer 7. The optical beam diameter is converted at a portion where the film thickness of the core layer 8 is changed. Further, since the light excited in the MQW layer 6 is confined more weakly in the mode conversion region 10 than in the gain region 9, a near field pattern at a taper bottom end portion is spread. As a result, a far field pattern which is a diffracted pattern of the near field pattern is contracted. Accordingly, an output angle of a beam emitted from the taper bottom end is narrowed, which makes coupling with an optical fiber easier. Three coneshaped patterns in FIG. 1C show intensity distribution of a photoelectric field.
Now, the core layer 8 shown in FIG. 1C has a structure in which an InGaAsP layer 8a and an InP layer 8b are laminated alternately as shown in enlarged view the FIG. 1D, and the InGaAsP layers 8a and 8b are applied with patterning stepwise using the InP layer 8b as an etching stop layer, thus varying the film thickness of the core layer 8 stepwise.
Since it means that patterning is repeated many times to vary the film thickness of the core layer 8 by such a method, the throughput is lowered.
When such a semiconductor waveguide is integrated in a monolithic manner with a light emitting element, active layers of the core layer 8 and the light emitting element become the same. Therefore, a defect is liable to be produced in the crystal of the active layer of the light emitting element in processing for varying the film thickness of the core layer 8, thus causing a fear that the characteristics of the light emitting element are deteriorated. Further, when the semiconductor waveguide and the light receiving element are integrated in a monolithic manner, a defect is also liable to be produced in the absorption layer of the light receiving element, thus also causing a fear that the characteristics of the light receiving element are deteriorated.
The FP-LD described in the above-mentioned citation [3] is structured by forming a first InP cladding layer 102, a waveguide layer 103, a multiple quantum well (MQW) active layer 104 and a second InP cladding layer 105 one upon another on an InP substrate 101 as shown in FIG. 2. The MQW active layer 104 is structured of an InGaAs well layer and an InGaAsP barrier layer, and is formed only in a gain region 110. Further, the waveguide layer 103 is formed in both of the gain region 110 and a mode conversion region 111. Besides, a reference numeral 106 represents a contact layer formed on the second InP cladding layer 105, and 107 represents an etching stop layer formed in the second cladding layer.
The waveguide layer 103 in the mode conversion region 111 is formed in a tapered form in the thickness direction, and gets thinner as becoming more distant from the gain region 110. The waveguide layer 103 has such a structure that an InGaAsP layer 103a and an InP layer 103b are laminated alternately. Further, the InP layer 103b is used as an etching stop layer, and the InGaAsP layer 103a and the InP layer 103b are applied with patterning to form a step form while changing the etching proof mask in a plurality of times, thus forming the film thickness of the waveguide layer 103 in the mode conversion region in a tapered form.
As to the light excited in the MQW active layer 104 and conducting through the waveguide layer 3, the near field pattern thereof at the taper bottom end portion spreads since optical confinement is weaker in the mode conversion region 111 than that in the gain region 110, thus resulting in that the far field pattern that is a diffracted pattern of the near field pattern is narrowed. Accordingly, the output angle of a beam emitted from the taper bottom end, thus making coupling with an optical fiber easier.
Besides, in the light intensity-current characteristic in the provisional publication [4], only the threshold current of 42 Ma and the differential quantum efficiency of 0.15 mW/Ma are obtained.
Now, the waveguide layer 103 forming a resonator has such a construction that the number of layers becomes less as getting near the output end, and the tapered form of the waveguide layer 103 is obtainable by changing the etching-proof mask in a plurality of times. Therefore, the construction is liable to include a crystal defect and deterioration of characteristics of a light emitting element is liable to be produced.
Further, what is important when a tapered waveguide layer is integrated in a semiconductor laser is to arrange so that the waveguide located in the mode conversion region does not act as an absorbing medium for an oscillation light.
Since the active layer 104 in the gain region 110 and the tapered waveguide layer 103 in the mode conversion region 111 are different layers and formed of almost the same composition in the construction described in the above-mentioned citation [3], a part of the laser light is liable to be absorbed in the portion near the gain region 110 of the tapered waveguide layer 103.
Accordingly, lowering of an optical output and a slope efficiency (a differential value of a leading edge of a current to light intensity characteristic curve) is unavoidable as compared with a laser element in which the mode converter is not integrated. In particular, since the tapered waveguide is located in a resonator in the FP-LD described in the citation [3], influence is exerted even on a fundamental characteristic of the semiconductor laser, and a threshold current in pulse measurement rises up to 70 mA in an element of double end cleavage. Moreover, no report has been made that continuous oscillation has occurred at room temperature in an element of double end cleavage.
Further, an apparatus in which a semiconductor laser and a tapered waveguide are integrated has been disclosed in [4] Japanese Patent Provisional Publication No. SHO63-233584 and [5] Japanese Patent Provisional Publication No. SHO64-53487. In the constructions thereof, the laser active layer and the tapered waveguide are also formed of the same composition, and the absorption loss in the taper region is great. In order to deny the absorption loss, it is required to inject a large current into the whole tapered waveguide, too.
In the light intensity-current characteristic in the provisional publication [4], only the threshold current of 42 mA and the differential quantum efficiency of 0.15 mW/mA were obtained as it is apparent from the description. Further, since the tapered form of the waveguide layer is obtained by devising the etching method, it becomes difficult to form the tapered form always uniformly, thus generating a fear that the yield is lowered in addition to poor controllability of the beam spot configuration. Moreover, a crystal defect is liable to be produced in the waveguide layer forming the resonator and deterioration of characteristics of a light emitting element is liable to be generated. Furthermore, the waveguide layer is composed of a single material, and has not an MQW layer in a tapered form such as described in the citation [3].
Besides, an optical semiconductor device having a tapered form in a lateral direction is described in the provisional publication [3], and processing in submicron order is required at the bottom end portion of the tapered waveguide. Hence, it is difficult to produce the same tapered form with good reproducibility because of the structure thereof.